Even perpetual calendars capable of keeping time over decades will need manual adjustment tonight as a leap second is added to 2016.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time, or UT1.
Without such a correction, time reckoned by Earth’s rotation drifts away from atomic time because of irregularities in the Earth’s rate of rotation.
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Since this system of correction was implemented in 1972, 26 leap seconds have been inserted, the most recent on June 30, 2015 at 23:59:60 UTC, and the next leap second will be inserted on December 31, 2016, at 23:59:60 UTC.